我的快乐秘诀

Since the lake opened, I’ve been swimming almost every day.
自从公众开放以来湖中有, 我一直在游泳几乎每天 

（自从湖对公众开放以来，自从……以来 means "since" ，公众 means "the public"，I think “对” means "for", e.g. “对我” means "for me"）

(我几乎每天在游泳。“几乎每天” and “一直” express the same meaning.)

Today, temperatures soared into the 80’s (29.4 celsius). I swam close to a mile, getting me closer to my goal.
今天，气温飙升至85（29.4摄氏度)，而我游泳近1英里（1.4公里）带我更接近我的目标

（，这让我更接近我的目标。 If you use “带我更接近我的目标”, it lacks subject, you should say “subject+带我更接近我的目标”，if you can say “subject+使我更接近我的目标” is better!）

My muscles are really sore, but exercise helps me to stay happy
我的肌肉是真的疼, 但运动可以帮助我保持快乐

（我的肌肉是真疼！ → native speaker /genuine/from the place noted for the product. You understand?）

This picture is cute! I am curious who takes the photo for you???

我的快乐秘诀

Since the lake opened, I’ve been swimming almost every day.
自从公众开放以来湖潭被開啓之後中有, 我一直在游泳几乎每天在游泳.

Today, temperatures soared into the 80’s (29.4 celsius). I swam close to a mile, getting me closer to my goal.
今日天[not wrong but I don't like same words appearing so close unless for emphasis]，天气温飙升騰達至85度（29.4摄氏度) .而我游泳近差不多游了1英里（1.4公里）带我更接慢慢达近我的目标 的 [目标  = target]

My muscles are really sore, but exercise helps me to stay happy
我的肌肉真的酸疼, 但能得运动可以帮助就令我保持心神愉快乐.

我的快乐秘诀 我快乐的秘诀

Since the lake opened, I’ve been swimming almost every day.
自从公众开放以来湖中有, 我一直在游泳几乎每天

这个湖（对公众）开放以后，我几乎每天都来这儿游泳。

Today, temperatures soared into the 80’s (29.4 celsius). I swam close to a mile, getting me closer to my goal.
今天，气温飙升至85（29.4摄氏度)，而我游泳近1英里（1.4公里）带我更接近我的目标
（即使）今天气温骤升至80几度（29.4摄氏度），我（也）游了快1英里——离我的目标更近了。

My muscles are really sore, but exercise helps me to stay happy
我的肌肉是真的疼, 但运动可以帮助我保持快乐

虽然现在我的肌肉很疼，但是运动可以让我心情愉悦：）

TIPs

I guess you can use more associated words in your essay，like however，therefore，whereas despite，apart from and etc.Because it will make your diary more smooth and logical.In chinese，it will be 即使……也 and 虽然……但是 in your essay，or others like 哪怕……也要，一方面……另一方面，尽管如此……可是，不论……还 etc.

I hope it will help you：）

我的快乐秘诀

Since the lake opened, I’ve been swimming almost every day.
自从开放湖中游, 我一直在游泳几乎每天

Today, temperatures soared into the 80’s (29.4 celsius). I swam close to a mile, getting me closer to my goal.
今天，气温飙升至85（29.4摄氏度)，而我游泳近1英里（1.4公里）带我更接近我的目标

My muscles are really sore, but exercise helps me to stay happy
我的肌肉是真的疼, 但运动可以帮助我保持快乐

我的快乐秘诀

Since the lake opened, I’ve been swimming almost every day.
自从湖对外开放以来, 我几乎每一天都在游泳。

Today, temperatures soared into the 80’s (29.4 celsius). I swam close to a mile, getting me closer to my goal.
今天，气温飙升至85（29.4摄氏度)，而我游泳近1英里（1.4公里），这让我更接近我的目标

My muscles are really sore, but exercise helps me to stay happy
我的肌肉是真的疼, 但运动可以使我保持快乐